5,447 research outputs found
Dynamics for Systems of Screw Dislocations
The goal of this paper is the analytical validation of a model of Cermelli
and Gurtin for an evolution law for systems of screw dislocations under the
assumption of antiplane shear. The motion of the dislocations is restricted to
a discrete set of glide directions, which are properties of the material. The
evolution law is given by a "maximal dissipation criterion", leading to a
system of differential inclusions. Short time existence, uniqueness,
cross-slip, and fine cross-slip of solutions are proved.Comment: 35 pages, 5 figure
Impact of Screw and Edge Dislocation on the Thermal Conductivity of Nanowires and Bulk GaN
We report on thermal transport properties of wurtzite GaN in the presence of
dislocations, by using molecular dynamics simulations. A variety of isolated
dislocations in a nanowire configuration were analyzed and found to reduce
considerably the thermal conductivity while impacting its temperature
dependence in a different manner. We demonstrate that isolated screw
dislocations reduce the thermal conductivity by a factor of two, while the
influence of edge dislocations is less pronounced. The relative reduction of
thermal conductivity is correlated with the strain energy of each of the five
studied types of dislocations and the nature of the bonds around the
dislocation core. The temperature dependence of the thermal conductivity
follows a physical law described by a T variation in combination with an
exponent factor which depends on the material's nature, the type and the
structural characteristics of the dislocation's core. Furthermore, the impact
of the dislocations density on the thermal conductivity of bulk GaN is
examined. The variation and even the absolute values of the total thermal
conductivity as a function of the dislocation density is similar for both types
of dislocations. The thermal conductivity tensors along the parallel and
perpendicular directions to the dislocation lines are analyzed. The discrepancy
of the anisotropy of the thermal conductivity grows in increasing the density
of dislocations and it is more pronounced for the systems with edge
dislocations
Dynamics of screw dislocations: a generalised minimising-movements scheme approach
The gradient flow structure of the model introduced in [CG99] for the
dynamics of screw dislocations is investigated by means of a generalised
minimising-movements scheme approach. The assumption of a finite number of
available glide directions, together with the "maximal dissipation criterion"
that governs the equations of motion, results into solving a differential
inclusion rather than an ODE. This paper addresses how the model in [CG99] is
connected to a time-discrete evolution scheme which explicitly confines
dislocations to move each time step along a single glide direction. It is
proved that the time-continuous model in [CG99] is the limit of these
time-discrete minimising-movement schemes when the time step converges to 0.
The study presented here is a first step towards a generalisation of the
setting in [AGS08, Chap. 2 and 3] that allows for dissipations which cannot be
described by a metric.Comment: 17 pages, 2 figures http://cvgmt.sns.it/paper/2781
A Multiscale Approach for Modeling Crystalline Solids
In this paper we present a modeling approach to bridge the atomistic with
macroscopic scales in crystalline materials. The methodology combines
identification and modeling of the controlling unit processes at microscopic
level with the direct atomistic determination of fundamental material
properties. These properties are computed using a many body Force Field derived
from ab initio quantum-mechanical calculations. This approach is exercised to
describe the mechanical response of high-purity Tantalum single crystals,
including the effect of temperature and strain-rate on the hardening rate. The
resulting atomistically informed model is found to capture salient features of
the behavior of these crystals such as: the dependence of the initial yield
point on temperature and strain rate; the presence of a marked stage I of easy
glide, specially at low temperatures and high strain rates; the sharp onset of
stage II hardening and its tendency to shift towards lower strains, and
eventually disappear, as the temperature increases or the strain rate
decreases; the parabolic stage II hardening at low strain rates or high
temperatures; the stage II softening at high strain rates or low temperatures;
the trend towards saturation at high strains; the temperature and strain-rate
dependence of the saturation stress; and the orientation dependence of the
hardening rate.Comment: 25 pages, 15 figures, LaTe
Void-induced cross slip of screw dislocations in fcc copper
Pinning interaction between a screw dislocation and a void in fcc copper is
investigated by means of molecular dynamics simulation. A screw dislocation
bows out to undergo depinning on the original glide plane at low temperatures,
where the behavior of the depinning stress is consistent with that obtained by
a continuum model. If the temperature is higher than 300 K, the motion of a
screw dislocation is no longer restricted to a single glide plane due to cross
slip on the void surface. Several depinning mechanisms that involve multiple
glide planes are found. In particular, a depinning mechanism that produces an
intrinsic prismatic loop is found. We show that these complex depinning
mechanisms significantly increase the depinning stress
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